An Australian G4S guard has told of a standoff between asylum seekers and Papua New Guinea guards in a "tribal sort of mode" during the violence on Manus Island that resulted in the death of an Iranian man.

The unnamed former Australian soldier, who served in the army for 20 years in Iraq, Afghanistan and East Timor, was working for G4S at the detention centre and has made a submission, including photos and videos, to a Senate inquiry into the riot.

The man, who has sought to remain anonymous, describes in detail the initial flare-up on 16 February and says the expatriate guards had to "jump on" the asylum seekers to stop the PNG guards from kicking them.

"Initially, the PNG guards didn't stop kicking," he said. "I remember being struck many times but my adrenaline level was quite high so I didn't feel pain until later that night."

He estimated the days of violence, which culminated in the beating to death of 23-year-old Iranian Kurd Reza Barati, involved 100 PNG guards and 30 asylum seekers.

"This caused a Mexican stand-off situation," he said.

The man describes the behaviour of the PNG guards in his submission.

"The problem with the PNG guards was they went into a tribal sort of mode," he said. "They were in a trance-like state of mind and nothing was going to get in their way.

"They wanted to drag all the [asylum seekers] out and maybe not kill them but show them that ‘this is our country'."

The man said there was no crisis management plan at the centre and limited communication options to provide situational reports to managers about the incident. He also claimed a senior manager told the expatriate guards to withdraw halfway through the incident.

"We, the expat G4S guards stated ‘no, no we're not leaving'," he said. "We were the only thing stopping the PNG guards."

He said Indian, Sri Lankan, Rohingya and Asian detainees weren't involved in the clash, which mainly involved Iranian, Iraqi and Middle East-based groups.

"The thing that absolutely pissed me off the most was officers had previously warned duty operation managers of the problems ... but this had fallen on deaf ears," he added.

The guard told of ripping the shirt off one asylum seeker, whose throat had been slashed, to stop the bleeding.